In 3GPP (3rd Generation Partnership Project), the packet-switched communication systems HSPA (High Speed Packet Access) and LTE (Long Term Evolution) have been specified for wireless transmission of data packets between User Equipments and radio base stations in cellular/mobile networks. In this description, the term “User Equipment” (UE) will be used to denote any suitable communication terminal adapted to communicate with a radio base station. A UE, may be implemented as a mobile phone, a PDA (Personal Digital Assistant), a handheld computer, a laptop computer, etc. A “radio base station” may be implanted as a NodeB, an eNodeB, a repeater, etc.
A base station serving a cell in a wireless network may transmit user data and control data in a physical downlink channel to a UE, and a UE may likewise transmit data and control data in a physical uplink channel in the opposite direction to the base station.
With the emergence of telecommunication services, so called heterogeneous networks have been developed. In heterogeneous networks there is a mix of differently sized cells served by base stations, such as macro, micro, pico, femto, and other type of devices such as relays and repeaters. For instance, a UE that is located within the coverage area of a macro radio base station, or partially within the coverage of the macro base station, may be served by a micro radio base station, which coverage area is a subset of the coverage of the macro radio base station. For example, even if an office building or department of a company is located within the coverage area of a macro base station, UE's of the employees will be served by an own micro radio base station of the company when being present in the office building/department. When being present outside the office building/department, the UE's of the employees are instead served by the macro base station. Typically, UE's for visitors to the office are not served by the micro radio base station, but are instead served by the macro base station.
In this description, the term “macro base station” is used to denote the radio base station which serves a larger “macro cell”, which overlaps or partially overlaps a “micro cell”. The term “micro base station” will be used to denote the radio base station which is serving the micro cell. Moreover, the terms “micro cell” and “micro base station” shall be interpreted generic, and are in this description used to denote any communication cell, and its respective radio base station, which is overlapped or partially overlapped by a macro cell. Thus, the terms micro cells and micro base stations are not limited to only micro cells and micro base stations, respectively, also pico cells, femto cells, and their respective radio base stations will be covered by these terms.
With “physical control channel” is meant physical channels which are used for communication of control data, e.g. scheduling initiations and decisions. In LTE, such physical control channels are e.g. PDCCH's (Physical Downlink Control Channel), and PUCCH's (Physical Uplink Control Channel)), which are used for communication of control data in downlink and uplink, respectively. With “physical shared channel” is meant physical shared channels which are used for communication of data. In LTE, such physical shared channels are e.g. PDSCH's (Physical Downlink Shared Channel), and PUSCH's (Physical Uplink Shared Channel), which are used for communication of data between communication terminals and radio base stations in downlink and uplink, respectively.
With reference to FIG. 1, a heterogeneous communication network will now be described. The micro cell 102 is served and controlled by a micro base station 104. One or more UE's 100 are served by the micro base station 104 and are scheduled by the micro base station 104 to receive downlink data from the micro base station 104. Scheduling information is sent on a downlink control channel 106, from the micro base station 104 to the UE 100, e.g. on a PDCCH. For instance, the scheduling information defines when downlink data will be sent. Downlink data is then sent on a shared channel 108 according to the scheduling information, e.g. on a PDSCH. The micro cell 102 is further embedded in a macro cell 112, and the macro cell 112 is served and controlled by a macro base station 114. One or more UE's 110 are served by the macro base station 114 and are scheduled by the macro base station 114 to receive downlink data from the macro base station 114. Scheduling information is sent on a downlink control channel 116 from the macro base station to the UE 110, e.g. on a PDCCH (Physical Downlink Control CHannel). Downlink data is then sent on a shared channel 118 according to the scheduling information, e.g. on a PDSCH (Physical Downlink Shared CHannel).
In the FIG. 1, communication of downlink data from the radio base stations 104, 114 to the UE's 100, 110 is described. However, communication of uplink data is performed similarly, even if the UE's 100, 110 requests their respective base station 104, 114 to schedule them to send the uplink data on uplink data channels (not shown in the figure), e.g. PUSCH's (Physical Uplink Shared CHannel). The requests are sent to the respective base station 104, 114 on uplink control channels (not shown in the figure), e.g. PUCCH's (Physical Uplink Control CHannel).
Typically, a heterogeneous communication network comprises various radio base stations having different capabilities and different downlink transmission power levels. For example, the output power difference between macro sites and micro or pico sites can be 10-20 dB, or even more. Therefore these radio base stations may have different coverage ranges. Hence the cells which they serve have different sizes, since the cell size is typically determined by the downlink transmission power, particularly on the reference symbols sent by the base stations.
In the uplink, the transmission power is, in principle, independent of the base station type and depends only on the UE (User Equipment) power capability. In the uplink, the UE should ideally be served by the base station which receives the strongest signal power from the UE, which typically is the base station with the shortest distance to the UE. Hence in the uplink the cells size should ideally not be based on the base station's power capabilities.
Hence in heterogeneous communication networks with smaller cells (micro, pico, femto, etc.) being overlapped or partially overlapped by a larger cell (macro), the optimum cell border may be different in downlink and uplink, and the optimal coverage area for the micro cell will be smaller in downlink. For instance, a UE which is located to a micro cell for best uplink coverage and served by a micro base station will receive signalling information (control data) from the micro base station. Generally, micro base stations transmit signalling information with a lower output power than macro base stations. Thus, even if the UE is served by the micro base station and receives signalling information from the micro base station, signalling information from the macro base station will reach the UE in downlink, and because the signalling information is arranged on the same resource elements for all UE's the signalling information, affect the reception of signalling information from the micro base station with interference. Especially, the signalling information to UE's located near the cell border where the macro signalling can be received with higher power level in case of uplink optimal cell association will be affected by interference. For signalling information in uplink, the interference problem will be less severe, due to the fact that the UE sends the signalling information with a substantially lower output effect, and the signalling information from the UE will not reach the macro base station, or be so weak that it does not affect signalling information substantially from other UE's served by the macro base station. Moreover, the user data in uplink and downlink, respectively, may be arranged in such a way that user data for a specific UE will not be affected by user data for other UE's. To arrange the user data, methods as Inter-Cell Interference Coordination (ICIC) may be applied. However, in LTE ICIC is only possible to use on shared channels for user data where frequency resources are orthogonal and not for signalling information on control channels such as PDCCH.
Interference of signalling information is not a major problem if the small and the large cells operate on different radio resources, for instance in a GSM network, because the performance degradation is not severe if the downlink cell association is used also for uplink communication. However, the difference between optimum cell size in the downlink and in the uplink is generally a problem if the two cells operate on the same radio resources (same frequency), for instance in VIE. The reason for this is that the smaller base stations typically use a lower output power than the overlaying macro base station, and the UE's use the same or similar output power. Furthermore, the interference situation for the control channels may be difficult. For example, the PDCCH from a smaller base station may be severely interfered by an overlaying base station.
It is thus generally a problem to provide reliable conditions for the control channels in overlapped cells in heterogeneous communication networks using shared radio resources.